There is conventionally known a fuel vapor treatment apparatus in which a fluid flow is generated by a pump in a purging passage for introducing fuel vapor produced in a fuel tank to an intake system for an internal combustion engine, forcibly purging the fuel vapor (for example, refer to JP-2002-332921A, U.S. Pat. No. 6,695,895). In such a fuel vapor treatment apparatus, in order to avoid the state where an air-fuel ratio control for the engine is complicated due to the fluctuation of a purge concentration of the fuel vapor, it is desirable to stabilize the purge concentration.
In addition, there is known a fuel vapor treatment apparatus which generates a fluid flow by an intake vacuum in an internal combustion engine in a fuel vapor passage where the fuel vapor produced in a fuel tank flows and at the same time, detects a physical quantity such as a flow quantity in correlation with a fuel vapor concentration in the fuel vapor passage, calculating a fuel vapor concentration from the detection result (for example, refer to JP-5-18326A). In such a fuel vapor treatment apparatus, in order to implement a prompt concentration measurement, it is desirable to stabilize the fuel vapor concentration in the fuel vapor passage.
However, in the apparatus for communicating a purging passage with a fuel tank through a canister, which is disclosed in JP-2002-332921A, since the concentration of the fuel vapor desorbed from the canister and flowing into the purging passage changes with time in response to a remaining fuel adsorption quantity in the canister, it is difficult to stabilize the purge concentration. Further, when the pump is located as exposed to an outside, an operating sound of the pump may be the cause of noises.
In the apparatus for communicating a fuel vapor passage with a fuel tank through a canister, which is disclosed in JP-5-18326A, since the concentration of the fuel vapor desorbed from the canister and flowing into the fuel vapor passage changes with time in response to a remaining fuel adsorption quantity in the canister, it takes time to measure the purge concentration. Yet, since the detection quantity is easily changed due to the fluctuation of an intake vacuum, this makes it more difficult to promptly carry out the concentration measurement.